1. Field of the Invention
The present invention relates to a pad slider caliper disc brake of the type which the current state of the art is, in essence, illustrated in FIGS. 1 and 2 of the accompanying drawings. In this type of brake, the pad back, or backing plates are slidingly fitted to an anchor bracket and a caliper housing is supported on the pad back plates.
2. Description of the Prior Art
In FIGS. 1 and 2 anchor bracket 20 is constructed for mounting to a motor vehicle. This has on it two slideways 21 which may typically be as drawn and which retain the ends of inboard back plate 23 and outboard back plate 24 so as to be slidable along the length of said slideways. To the inboard back plate 23 a friction pad 25 is attached and to the outboard back plate a friction pad 26 is attached. A caliper housing 27 includes a cylinder 28 at its inboard end which accommodates a piston 29, the outer face of piston 29 being attached to the inboard back plate 23. Extending from cylinder 28 is a caliper bridge 31 terminating in fingers 32 which are attached to the outboard back plate 24. Backplates 23 and 24 will typically be spring loaded to prevent rattling in service for example by two springs 34 and 35.
It is emphasized that FIGS. 1 and 2 are purely diagrammatic and are included only to illustrate the current state of the art.
Whilst the advantages of a pad slider caliper are known there are disadvantages with current designs which this invention seeks to improve upon. These disadvantages are described below.
The principal disadvantage that the present invention assists in overcoming is that the captive nature of the abutments of both back plates means that the housing/pad assembly cannot be simply unclipped and removed from the anchor bracket. In general, the most practical means of servicing the caliper, e.g., replacement of worn pads, is to unbolt the anchor bracket from the vehicle, remove it from its position over the disc 33 then proceed to dismantle the pads from the caliper housing. This problem is compounded by the fact that the inboard pad back plate is attached or keyed to the piston in some way, as is the outboard pad back plate to the fingers of the housing, thus making it difficult if not impossible to remove the caliper housing from the pads in situ.
Further disadvantages that are mitigated by optional features of the invention are firstly, the method of pad retention in general makes it difficult to design an anchor bracket with an integral outboard tie bar (not shown) because the limited space availability on many modern cars, precludes the incorporation of a tie bar and provision of sufficient clearance for pad removal. The anchor bracket must therefore be more robustly built in order to provide adequate stiffness, thus adding to the mass of the assembly.
Secondly, in current designs there is little provision for the take up of the accumulation of dimensional tolerances in the various components which could cause housing misalignment and consequent uneven pad wear and brakes-off drag. The prime cause of this is the lack of compliance in the connection between the inboard back plate and the piston and/or the connection between the outboard back plate and the caliper housing fingers. In general, such means is substantially rigid in a plane perpendicular to the piston axis. Hence, the caliper housing is forced to take up a position dictated by the sum of the tolerances of the interrelated components of the brake assembly and consequently will not necessarily be in correct alignment relative to the braking surfaces of the disc. Uneven, taper pad wear and brakes-off drag result.
Thirdly, in general, the spacing of the two pad back plates is narrow, relative to the overall depth of the caliper housing and becomes narrower as the pads wear. Due to the shape of the housing and the location of the piston, the center of gravity, of the housing assembly is offset inboard of the supporting back plates. The resulting cantilever effect tends to tilt the housing/pad assembly, with the resultant possibility of taper wear and brakes off drag.